1. Field of the Invention
The present invention relates to a crack resistant magnetic write head and more particularly to a write head with an enlarged overcoat layer portion adjacent an air bearing surface (ABS) for absorbing stress arising from impacts and heat.
2. Description of the Related Art
A typical magnetic write head has one or more coil layers embedded in an insulation stack that is sandwiched between first and second pole pieces. The first and second pole pieces are magnetically connected and have first and second pole tips separated by a magnetically insulative gap layer to form a gap at an air bearing surface (ABS). The coil layer induces a magnetic field in the pole pieces which fringes across the gap. The magnetic write head is covered with an overcoat layer for protection of the aforementioned components.
A combined head, such as a merged magnetoresistive (MR) head, includes the aforementioned write head as a write head portion combined with an MR read head portion. The combined head is carried on a slider that is mounted on a suspension in a magnetic disk drive. The suspension is mounted to an actuator which moves the combined head over selected tracks on a rotating disk for reading and writing signals thereon. As the disk rotates, a cushion of air provides an air bearing between the disk and the slider which counterbalances the loading force exerted by the suspension. The ABS, the surface of the slider facing the disk, is typically spaced about 0.075 .mu.m from the disk when the disk rotates. When the rotation commences, the slider takes off from the disk and rides on the cushion of air. When the disk stops, the slider lands on the disk. This is referred to in the art as contact start and stop (CSS) operation. Friction and heat are developed by both events.
In the fabrication of a thin film magnetic write head it is important that zero throat height (ZTH) be accurately located. The ZTH is the point after the ABS where the first and second pole pieces separate from one another. This separation is caused by a central recessed edge of the insulation stack which elevates the second pole piece above the first pole piece. It is desirable to locate the ZTH as near the ABS as possible in order to minimize flux leakage between the pole pieces. The insulation stack typically includes a first insulation layer (I1) on the first pole piece layer, one or more coil layers on the first insulation layer, a second insulation layer (I2) over the coil layer and a third insulation layer (I3) over the second insulation layer. One of these insulation layers, typically the first insulation layer, has a forward central edge which extends laterally and parallel to the ABS and defines the ZTH. This central edge is typically located about 1-2 .mu.m from the ABS.
A portion of the ABS is formed by front edges of the thin film components of the write head. These edges comprise the front edge of the gap layer and the front edges of the first and second pole tips. An ABS view of the write head shows a small gap layer end disposed between a wide first pole tip end and a narrow second pole tip end. The second pole tip end appears as a pedestal with first and second side walls that are parallel to one another and perpendicular to the ABS. The second pole tip is the trailing pole tip with respect to a rotating disk. Accordingly, the lateral width between the side walls of the second pole tip defines the track width of the write head. Surrounding the edges and forming a portion of the ABS is a front edge of the overcoat layer. The overcoat layer has a wide lateral expanse in both directions from the first and second walls of the second pole tip. The overcoat layer also extends rearwardly toward the back gap, interfacing with the central edge of the ZTH-defining insulation layer, which typically is the first insulation layer. This interface extends parallel to the ABS along substantially the full lateral width of the front edge of the ZTH-defining insulation layer. Since the edge of the ZTH-defining insulation layer is recessed only 1-2 .mu.m from the ABS, there is an extremely small volume of the overcoat layer between the ABS and the edge of the ZTH-defining insulation layer.
A magnetic write head is a lamination of many components. Unfortunately, these components are fabricated from different materials with different thermal coefficients of expansion and different moduli of elasticity. Typically, the insulation stack is photoresist, the overcoat layer and the gap layer are alumina (Al.sub.2 O.sub.3), the pole pieces are Permalloy (Ni.sub.79 Fe.sub.21), and the coil layer is copper. The modulus of elasticity of the photoresist is 7, as compared to 100 for alumina and 200 for Permalloy. Therefore, relative to the overcoat layer and the pole pieces, the insulation stack is very compliant. Further, the coefficient of thermal expansion of photoresist is 36 E-6/degree C., whereas the thermal expansion of the overcoat layer is 6 E-6/degree C. Accordingly, the insulation stack expands 6 times as much as the overcoat layer.
The write head is a delicate structure which, unfortunately, is subjected to impact shock during fabrication, during assembly in a disk drive, during shipment and during use. Further, the head is subjected to expansive stress due to heating of the coil. During contact start and stop, the ABS end of the head is subjected to both heat and impact loading. The head components are stressed and strained by these conditions. The weakest region of the head is the small volume of the overcoat layer between the ABS and the front edge of the ZTH-defining insulation layer. An alumina overcoat layer absorbs stress well, especially since its modulus of elasticity is reasonably close to that of the Permalloy pole tips. Unfortunately, the extent of the alumina overcoat layer between the ABS and the front edge of the ZTH-defining insulation layer is minimal. Impact loading at the ABS quickly affects the insulation stack, which responds like jelly as compared to the other layers. Further, when heated, the insulation stack expands more than the overcoat layer, causing additional stresses and strains at all interfaces with the insulation stack. It would be desirable to increase the depth of the overcoat layer from the ABS into the head, but this is not practical because the magnetics of the head would be adversely affected by an increase in the ZTH.
Because of the structure and the materials employed, the write head develops cracks at the ABS. The weakest point at the ABS is the inside corners of the second pole tip. Cracks typically radiate from these corners and migrate laterally or radiate at some lateral location and migrate toward these corners. With repeated stress, these cracks grow. This phenomenon and resultant degradation of head performance is reported in IEEE Transactions on Magnetics, Vol. 31, No. 6, November 1995, page 2991 by Chekanov et al.